Specifically, the invention relates to the processing of melts of glass, metals, semimetals and similar materials at high temperatures of several 100° C. The processing can comprise, for example, production of a melt of raw material, in the case of glass especially crushed glass and/or mechanical mixtures, and also refining, homogenization and further steps such as passing to shaping. In all such process steps, use is made of vessels such as crucibles, melting tanks, refining tanks, pipes, stirrers and other apparatuses of which areas come into contact with the melt during operation.
A material for such components has to have a high resistance to chemical attack as a result of contact with the melt, i.e. a high corrosion resistance, so that contamination of the melt by introduction of particles of the material is largely avoided.
In DE 102 44 040, the applicant proposes a sintered silica material which has a two-layer structure for glass production. Cristobalite is a crystal modification of SiO2 and offers the advantage of higher corrosion resistance than amorphous fused silica. According to DE 102 44 040, care has to be taken in the production of the material by slip casting of fused silica particles to ensure that no or only little cristobalite is formed, since beta-cristobalite transforms into alpha-cristobalite at about 270° C. with a volume decrease of at least 2%. This volume change is considered as destructive to the component.
To be able to utilize the high corrosion resistance of crystalline SiO2 during use of the component, the particle size of the SiO2 grains is, at least in the outer region of the component, selected so that cristobalite formation occurs more quickly than removal of material from the surface of the fused silica as a result of corrosion during operation. An outer layer of the material for use in contact with a glass melt comprises fused silica grains having a diameter of less than 40 microns. These small fused silica grains are used so that very rapid devitrification of the fused silica with conversion into cristobalite occurs during operation at appropriately high temperatures.
However, the production of a two-layer shaped body is labour-intensive and therefore costly. In addition, the sintering process has to be carried out at sufficiently low temperatures for crystallization to be largely avoided. The sintering process is therefore time-consuming. Furthermore, when the shaped body is used in contact with a melt during operation at high temperatures, the shrinkage which then occurs as a result of transformation of the amorphous fused silica into crystalline modifications has to be taken into account.